PHOTODISSOCIATION AND UV RADIATIVE-TRANSFER IN A CLOUDY ATMOSPHERE - MODELING AND MEASUREMENTS

We present an analysis of UV radiative transfer under cloudy condition s in relation to the photochemistry of the hydroxyl radical (OH) by me ans of modeling and measurements. The measurements, which are part of the First Aerosol Characterization Experiment (ACE 1) campaign, consis t of four different ascents/descents selected from research flight (RF ) 12 and 28. The ascents/descents give vertical profiles of UV irradia nces, microphysical properties, standard meteorological parameters, an d OH concentration in the presence of one or more cloud layers, In ord er to assess the photochemical conditions for these (complex) cloudy c ases we first comp.are the UV irradiance measurements with modeled pro files. We model the UV irradiances using cloud optical properties whic h we derive from the measured microphysical properties. Second, we use the simultaneously modeled actinic flux to calculate the rate constan t of the photodissociation of ozone to the O(D-1) radical (J(O3)) This reaction initiates the primary OH-production. Finally, we compare the measured OH concentrations with those derived from the radiative tran sfer calculations. For single-cloud layer cases we successfully simula ted UV radiative transfer, J(O3) and OH. For more complex multiple-clo ud layer cases the UV radiative transfer could only be explained allow ing large variations of the cloud optical thickness (from zero to doub le the measurement derived values). The impact of such variations on t he modeled radiation-derived photochemical properties, J(O3) and OH, w as, with a variation of 25%, found to be relatively small. As a conseq uence, we were able to simulate the general profile of OH for these co mplex cloudy conditions.